Urethanes have been prepared by the reaction of an alcohol with an aromatic isocyanate. The isocyanate was obtained by the reaction of phosgene with the corresponding primary amine which was prepared by reduction of the corresponding nitro compound. This conventional process has various disadvantages, including the toxicity and corrosive nature of phosgene and the formation of hydrogen chloride as a by-product. Moreover, certain aromatic amines have harmful biological properties and some of them tend to be oxidized by air in storage.
There have therefore been attempts to avoid the use of the highly toxic phosgene and to prepare urethane directly from the corresponding nitro compounds, alcohols and carbon monoxide. The processes according to U.S. Pat. No. 3,993,685 and German Offenlegungsschrift No. 2,603,574 use catalyst systems based on metals of the platinum group. Since these processes inevitably entail considerable losses of these highly expensive catalysts, they have hitherto not become widely used on a commercial scale.
In the process according to German Offenlegungsschrift No. 2,343,826, it is proposed to use a combination of selenium or sulphur, or compounds of these elements, with very large quantities of a base as the catalytically active system. The bases used include, for example, triethylamine and pyridine. In order to be able to start the reaction satisfactorily in the presence of these amines, however, it appears to be necessary to use them in relatively large quantities compared with the nitro compound used as starting material. If dinitrotoluene is used as the nitro compound, the quantity of amine used is equal to, or greater than, that of dinitrotoluene. The use of such large quantities of amine entails numerous problems of an economical nature and particularly with regard to the recovery process. Moreover, this process leads to the formation of by-products such as amino compounds and ureas if measurable quantities of water are present, e.g. as hydrates or in the free form. The process according to German Offenlegungsschrift No. 2,343,826 is therefore also for the most part unsuitable for use on a commercial scale.
In the process according to German Offenlegungsschrift No. 2,614,101, the reduction in yield of the desired urethanes due to the above-mentioned formation of by-products can be avoided by using a catalyst system which is composed of elementary selenium or a selenium compound and a promoter consisting e.g. of a bicyclic amidine and a carboxylic acid. Although the process according to German Offenlegungsschrift No. 2,614,101 enables higher yields of urethanes to be obtained than in the process according to German Offenlegungsschrift No. 2,343,826, it also gives rise to troublesome quantities of by-products due, in particular, to hydrolysis and secondary reactions of the urethane formed.
The process according to German Offenlegungsschrift No. 2,623,694 is regarded as a further development of the process according to German Offenlegungsschrift No. 2,614,101 in that the formation of by-products of urethanes by the use of aromatic amino compounds or aromatic urea compounds corresponding to these by-products is suppressed. Although this measure provides an improvement to the process according to German Offenlegungsschrift No. 2,614,101, the process according to German Offenlegungsschrift No. 2,623,694 still has serious disadvantages. In particular, it requires the use of large quantities of organic amidine salts which may cause trouble in working up the end products. Furthermore, the process of German Offenlegungsschrift No. 2,623,694 has the disadvantage of requiring the use of very large quantities of selenium or selenium compounds.
It was therefore an object of the present invention to provide an improved process for the production of urethanes from aromatic nitro compounds, alcohols and carbon monoxide which would not have the disadvantages mentioned above of the known processes and which, in particular, would enable the quantity of selenium or selenium compound required to be substantially reduced and would also enable, as far as possible quantitative urethane formation to be achieved in spite of the reduction in the quantity of catalyst used.